The present invention relates to well drilling apparatus and more particularly to drill collar stabilizers.
Drill collar stabilizers are used in directional drilling operations to control the angle of a bore hole being drilled from the surface. By means of drill collar stabilizers, a bore hole extending at an angle to the vertical may be drilled. Drill collar stabilizers may also be used for guiding a drill string and drill bit in alignment when drilling a vertically-extending bore hole.
To extend a bore hole at an angle to the vertical, stabilizers are commonly mounted between a series of drill collars. Drill collars are heavy elongated tubular members which can be deflected along their length during a drilling operation. The degree of deflection of an assembly of drill collars is controlled by the disposition of stabilizers with respect to a series of collars. The stabilizers which bear against the wall of the bore hole act as fulcrums to enable an assembly of drill collars to be flexed about the fulcrums. Thus, the spaced location of the stabilizers on the drill collars in conjunction with the weight or force of the drill collars determined the degree of deflection of the assembly of drill collars. Not only is the degree of deflection of the drill collars directly adjacent the drill bit controlled by stabilizers, but also the degree of deflection of other drill collars spaced along the bore hole.
It has become common to drill to greater and greater depths for oil and gas and to employ directional drilling i.e. drilling at an angle to the vertical. Correspondingly, drilling operations and equipment have become more sophisticated and complex, particularly offshore drilling operations and equipment. For example, a plurality of bore holes may be drilled from an offshore drilling rig secured at a single location with each hole extending in a different direction. This is accomplished by directionally drilling a plurality of bore holes at different angles with the vertical and in different directions. The holes are relatively closely spaced at the surface to enable the plurality of holes to be drilled from a single drilling rig location. As drilling proceeds, the angle of drilling of each bore hole is controlled in order that the bore hole locations and depths are spaced over a relatively large area in a grid-like pattern. The bore holes may also be directed to be in a plurality of strata, each of which may be at a different depth and location. To reach a plurality of different locations, it is necessary to control the drilling angle with extreme accuracy for each bore hole in order that the bottom of the bore hole is within as little as several feet of a predetermined location. The location at which stabilizers are disposed along an assembly of drill collars and the weight of the drill collars control the deflection of the assembly and thereby the drilling angle. Accordingly, it is necessary to position the stabilizers at a plurality of different positions along the length of the drill collars to control the drilling angle. Computers are used to determine the proper position of the stabilizers on an assembly of drill collars.
In order to control the drilling angle and direction of a bore hole, magnetic readings of the heading of the bore hole, i.e., readings based on the earth's magnetic field and movement of the indication of the angle of the bore hole, may be taken in the bore hole which, in conjunction with the depth at which the readings are taken, yield a spatial determination of the bore hole. To permit magnetic monitoring, the drill collars are made of non-magnetic material.
It is not uncommon to drill the depths of 14,000 feet or more. The weight of the drill string is supported by the rig at the surface but the drill bit is loaded vertically by the predetermined weight of a plurality of drill collars. Accordingly, considerable forces are encountered by the drill collars and the stabilizers during a drilling operation or during raising or lowering of the drill string. For example, axial forces in the range of 200,000 pounds may be applied to a stabilizer during the raising or lowering of an assembly of drill collars and torques in the range of 60,000 foot pounds may be encountered by the stabilizers during drilling. During directional drilling, a stabilizer acts as a fulcrum point between the wall of a bore hole and the assembly of drill collars. As a result, tremendous torques may be applied to a stabilizer. Accordingly, drill collar stabilizers must be constructed to withstand such forces and torques. In addition the wear surfaces of stabilizers must be made of a hard material such as steel, tungsten carbide, or the like.